Analyzing or monitoring the composition of gases or solutions in hostile environments such as a chemical reaction inside an autoclave or other suitable pressure vessel or the ambient atmosphere inside a machine pressure vessel frequently requires spectrophotometric techniques based on emission, absorption or scattering processes for qualitative and quantitative analysis. Only optical contact with the gases or solutions of the environment is required.
Frequently the environment is caustic or characterized by high temperature or high pressure and would destroy spectrophotometric analysis equipment such as lasers, filters and monochromators. Therefore the analysis equipment is best placed in a laboratory environment remote from the hostile environment. Probes or cells provide optical connection with the hostile environment and communicate with the analysis equipment via direct light paths or more typically, fiber optic cables. Where measurements must be made at many different locations, a number of probes may be connected to a single piece of remotely-located analysis equipment.
One typical spectrophotometric technique is Raman spectroscopy. When light of a single wavelength interacts with a molecule, the light scattered by the molecule contains small amounts of light with wavelengths different from the incident light. This is known as the Raman effect. The wavelengths present in the scattered light are characteristic of the structure of the molecule, and the intensity of this light is dependent on the concentration of these molecules.
A major difficulty associated with Raman spectroscopy is the low intensity of the Raman scattered light compared to the intensity of the exciting incident light. Additional and intrinsic to the use of fiber optic cables in Raman spectroscopy is the excitation of Raman scattering inside the optical fiber itself (commonly referred to as `silica scattering`), most notably in the fiber optic cable carrying the exciting high intensity laser light to the probe. Further the intensity of Rayleigh scattered light coming from the sample presents a very large background for the Raman signal. Finally stray light, that is light which is specularly or otherwise reflected back into the collecting fiber, may cause additional noise.
MacLachlan (U.S. Pat. No. 4,573,761), Sela (U.S. Pat. No. 5,381,237), and Schrader (U.S. Pat. No. 5,543,997) teach fiber optic probe heads designed to improve the collection of scattered light from the sample. The probes by Sela and Schrader are of the 180.degree. backscattering type, while the probe by MacLachlan allows for a scattering geometry slightly different from 180.degree..
MacLachlan's probe identifies a Raman probe that consists of a strand fiber bundle, where the central fiber excites the sample and typically six fibers surround the central fiber to collect the signal. This probe is not strictly of the 180.degree. backscattering type since MacLachlan suggests that the axes of the collecting fibers best make an angle of 10-20.degree. with respect to the axis of the central exciting fiber. The design of the probe, however, does not permit the angle between the axis of the exciting and the axes of the collecting fibers to be much greater than 10-20.degree.. Furthermore MacLachlan states that an angle of convergence beyond 45.degree. essentially reduces the performance of the probe to that of a 180.degree. backscattering arrangement. MacLachlan teaches an adaptation of the probe for insertion in and connection to a process vessel such as a reactor and proposes an appropriate shell with a window made of fused silica, sapphire or diamond. However, the design of the probe is incompatible with high temperatures.
The probe by Sela employing a gradient index lens teaches no adaptation of the design for use in a hostile environment and is not suitable for such applications.
The five probes by Schrader are intended for medical or biological applications and are not suitable for a hostile environment. The fiber optics used in the probes meet no requirements to withstand high temperatures and the window identified as glass, quartz or sapphire is not designed as a high pressure boundary.
Owen (U.S. Pat. No. 5,377,004) gives a comprehensive summary of prior art addressing the problem of silica scattering and Rayleigh scattering and means of filtering these noises. Owen teaches improved optical geometries using lenses, mirrors and optical filters to reduce both noise sources in a 180.degree. backscattering Raman probe. The patent teaches no application of this technology for a hostile environment. In fact the use of lenses, mirrors and optical filters requires that the probe is held near room temperature.
Nave (U.S. Pat. No. 5,404,218) addresses the problem of stray light reaching the collecting fiber (that is light which is specularly or otherwise reflected by the inside of the sample cell) in a 180.degree. Raman backscattering geometry and describes a cell coated on the inside with an anti-reflective layer as well as an optimal cell geometry to reduce the intensity of stray light coming from the walls of the cell. The cell is designed for sampling of possibly caustic gases or solutions as long as the antireflective coating on the inside of the cell is inert with respect to the analyte. The optical fibers used are not identified as being capable of withstanding high temperatures and a fiber optic window identified as sapphire or quartz is optional. However, sapphire and quartz material are subject to caustic corrosion attack and will probably have short useful lifetimes. No high pressure application is proposed. The probe by Nave is only of limited use in a hostile environment.
Weller (U.S. Pat. No. 5,046,854) describes two photometric cells adapted for use in a hostile environment using fiber optic cables and windows made of sapphire, cubic zirconia, diamond, ruby, glass, quartz, Suprasil or Infrasil. Only one of these cells has a design suitable for Raman spectroscopy. The first geometry suggested therein is of the 180.degree. backscattering type. The cell normally includes a mirror at the end face of the cell which is intended to reflect the incident light back into the collecting fiber. Weller, however, suggests that this cell may be adapted for Raman spectroscopy by omitting the mirror at the end face. In the second geometry the excitation laser light is directed straight at the collection fiber optic. This geometry is suitable for absorption spectroscopy, but is the most unfavorable design for any Raman spectroscopy. In a preferred embodiment diamond windows are brazed to the cells to create a pressure boundary, but this seal is likely to crack under thermal cycling.
Cowen (U.S. Pat. No. 4,682,846) describes a hermetic high pressure fiber optic bulkhead penetrator for use in submersible marine equipment, but Cowen makes no reference to a caustic or high temperature environment. The bulkhead penetrator is intended as an optical feedthrough for fiber optics. Nevertheless it could be used for a Raman probe, but no reference to any form of spectrophotometric application is made.
Previous spectrophotometric probes suitable for Raman spectroscopy generally detect light backscattered by 180.degree. into the collecting fiber. This geometry though less optimal is chosen, because it allows for a compact design of the probe. However, in a 180.degree. backscattering geometry Rayleigh scattering received by the collecting fiber is maximized which creates a large noise background for the Raman signal which tends to be orders of magnitude smaller.
These probes are often cells that contain the analyte or are optical fibers pressed against the sample. Some of these probes are designed for use in a hostile environment with windows creating a boundary against a high pressure or caustic environment, but few if any of these cells are suitable at high temperatures.